Once you have absentee ballots, requiring the same effort as voting to get, with a caveat for delivery to shut ins(as voting IS that important) I simply do not see how they are any more a source for fraud than anything else.

Non-absentee voting is designed to make it hard to buy votes. You can pay someone to vote a certain way, but when they go into the poll you have no way of knowing how they really voted. The voting machines where I vote are specifically designed so there's no photo you could take in the voting booth that shows how you voted. They're running somewhat behind technology, because you could show how you voted with a video recorded in the booth.

With absentee voting there's no protection whatsoever against selling votes, and you really have no way of knowing who actually cast the vote either.